The objective of this project is to carry out a prospective cohort study of diet in relation to several major cancers, especially breast, prostate and colorectal malignancies. The cohort will consist primarily of male and female members of the American Association of Retired Persons (AARP). In order to guarantee adequate heterogeneity of dietary intake for key nutrients and foods, the cohort will be established in a two-phase process. Food frequency questionnaires will initially be sent to 3.5 million AARP members. All respondents falling within the extreme categories of dietary intake (e.g., for fat, vegetables, or various 'dietary pattern' combinations of nutrients and foods) will be selected for the final cohort, along with a random sample of those falling within the intermediate categories of intake. The final cohort will consist of 350,000 persons, half men, half women. Oversampling of minority AARP members will be done to ensure an adequate representation of minorities in the final cohort. Validation/calibration studies of the food frequency questionnaire will be carried out within the first year of the contract period. A second questionnaire, comprising questions on exposures not assessed in the first questionnaire, will be mailed to cohort members several months after the initial questionnaire. A brief followup questionnaire, primarily targeted to endpoint assessment, will be mailed to cohort members at the end of the five year period of observation. Followup will be largely passive, through established state registries. The initial questionnaire mailing will be to AARP members in those states selected on the basis of having registries with adequate coverage and quality. Active followup with record retrieval will be conducted for the small percentage of cohort members who have moved out of the cancer registry areas.